Lurking among the icy moons of Saturn, the giant moon Titan looks like an eerie shadow of Earth. Hidden under a dense organic atmosphere are frigid lakes and black dunes made of hydrocarbons. Methane rain sometimes pours from the orange skies.

Now it seems this monstrous copy of Earth may actually have a similar background&colon; like Earth, Titan was built from several smaller bodies smacking together and merging to become a planetary chimera.

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The discovery hints that the creation of Titan also spawned icy minions – a family of moons with similar compositions that are between 7 and 29 per cent the size of Titan. No other planet in the solar system has such a grouping of midsized moons. Mighty Jupiter, for instance, skips right from its four largest satellites to hunks of rock no more than a twentieth of their size.

“One of the big mysteries is, how do you parcel out mass into equal-sized blobs and have them survive as satellites around one planet, then have another planet next door that has none of these midsized moons?” says Erik Asphaug of the University of California, Santa Cruz.

Collide and congeal

One explanation could be that the Saturn system evolved like a planetary nursery. Planets emerge from discs of orbiting material that coalesces into smaller bodies. Moons can probably form around planets in the same way.

Taking the analogy a step further, Asphaug and colleagues suggest that Titan could have taken shape just like a mini-Earth.

“We think the Earth formed from maybe 10 giant impacts, with planets merging to form yet-bigger planets,” says Asphaug. “We’re thinking of that same thing for the Saturn system, with Titan being the last champion, gathering up all the mass in the system.”

In their model, Saturn starts out with a family of four large moons, similar to Jupiter’s big Galilean satellites. Computer simulations show that, as these young moons settle into stable orbits, their mutual gravity forces them to fall towards each other and go through a series of slow collisions.

Dirty slush

Although the proto-moons hold lots of ice, slower impacts mean that their raw material would melt rather than vaporise, leaving behind a pliable, dirty slush that can re-form into a new proto-moon. As the final two proto-moons merge, they liberate some icy material from their mantles, which spins outward like flailing arms and then congeals into the midsize moons we see today.

“We can form Titan in a series of mergers, and each time you leave behind a couple of midsized moons,” says Asphaug.

He acknowledges that there are pieces of his model that still need to be validated, such as whether the small moons would definitely end up in their current orbits, and whether they would have the right compositions. But if the model holds, could such violent beginnings be the best way to stitch together worlds with atmospheres and lakes?